The use of electronic and computer systems has expanded immensely and affects almost all ways of life. Even brief interruptions of or variations in the input power source can cause failures to these systems, resulting in lost time, data, damaged equipment, and high repair bills. To overcome these problems, UPS systems have been developed and are well known in the art. These include both on-line and off-line configurations. The simplest type, single conversion or standby, uses a battery, an inverter to convert the battery voltage to an AC output voltage, and a transfer switch that connects the AC output voltage if the source voltage is lost. The transfer time during the switching can be noticeable and can become significant in some computer applications. Double conversion or on-line types eliminate this transfer time. The AC source voltage is continuously converted by a rectifier to a DC voltage and charges a battery that is in parallel with the DC voltage. This DC voltage is converted by an inverter back to an AC output voltage. If the AC source voltage is lost, the battery supplies the DC voltage to the inverter immediately and there is no interruption in power to the load. Upon the return of the AC source voltage, the load is again supplied from the AC source without any interruption in power. Since the incoming power is rectified, incoming power is inherently conditioned since surges, sags and noise can be essentially eliminated during the conversion to DC.
Many types of on-line UPS systems have been developed. Commonly assigned U.S. Pat. No. 4,916,329, describes one such device that uses a three port ferroresonant transformer having a first primary winding connected to an AC input, a second primary winding connected to a battery-inverter arrangement, and the secondary winding connected to the load. A series regulator in the first primary keeps the output voltage within certain limits when the AC input is present. The regulator also disconnects the AC line when the inverter supplies the output voltage. A synchronization circuit provides a means of having the AC output of the inverter in phase with the AC input when a transfer takes place to prevent large voltage transients.
Phase lock loops (PLL) have been commonly used as a means for locking one frequency to another to provide line synchronization and are commercially available as integrated circuit packages, such as a Harris Semiconductor type CD4046B. The possibility of a digital PLL and several algorithms are discussed in a master's thesis submitted by Thomas E. Helfrich to West Coast University and accepted on Apr. 11, 1991 entitled "Microcontroller Application In An Uninterruptible Power Supply". These algorithms are written in Basic and are not suitable for real time implementation. Compensation is required to eliminate observed frequency variations due to sampling times of the zero crossing detectors versus the actual zero crossings.
U.S. Pat. No. 4,719,550 describes another UPS system that is adaptable for use with alternate energy sources. This system requires an input source of given frequency and amplitude. A DC bus is established that is fed from either rectified AC input power or batteries. This is converted back to output AC voltage. Various configurations using different combinations of AC/DC, DC/AC, DC/DC and AC/AC converters are presented. Regulation of the AC output is controlled by keeping the DC bus at a predetermined energy level. Energy transfer into the bus is controlled by adjusting the magnitude and phase relationship of the AC input voltage and the AC side of an AC/DC converter feeding this bus.
These and other known types of UPS devices have various operational characteristics and features that are unique to the method employed and would be difficult to integrate into a single, cost effective device,. These include high efficiency in the normal mode of operation, small size, reliability of components, regulation of the AC output voltage during both on-line and standby modes of operation, fast transfer times between modes with low electrical noise generation, extended battery life, line isolation between source and load, synchronization of the input AC phase with the output inverter phase to eliminate possible voltage spikes to the load and the device during transfer times, and diagnostic capabilities. A microprocessor based UPS device could allow the integration of these features wherein the microprocessor eliminates the need for extensive hardware, with a reduction in power requirements, without compromising performance and increasing the overall efficiency of the UPS device.
Most output inverter stages employed in UPS systems compare the voltage output with a desired output in an error amplifier to produce an error signal proportional to the error. The error signal is then applied to the input of a pulse width modulator (PWM) operating at a frequency much higher than the output frequency. The width of the output pulse is modulated with respect to the error signal and applied to switching type amplifier. The output is filtered by an inductor to remove the high frequency components caused by the switching mode of operation. This may cause a problem in system frequency response and stability since the inductor introduces a pole in the system transfer function. To overcome this problem, an inner current feedback control loop is introduced to the voltage feedback to effectively eliminate the output inductor. These types of feedback systems utilized peak current mode control. This control requires slope compensation in the circuitry and requires compensation for peak to average current errors. To improve on this drawback, a current feedback system that relies on average current rather than peak current is desired. These types of control systems have been commonly developed for DC amplifiers but do not exist for AC systems.
Another problem commonly associated with prior art UPS systems is providing adequate protection for the solid state output devices to prevent failures due to overloads, short circuits and overheating.
Applicants' invention is provided to solve this integration problem and to provide other unique features that will become readily apparent.